1. Technical Field
This invention relates to a discharge-lamp control device for controlling a discharge lamp used, for example, as a backlight of a liquid crystal display.
2. Related Art
Liquid crystal displays are widely used as personal computer monitors and/or televisions as well as displays for portable personal computers and word-processors. Recently, as the liquid crystal displays have become larger in size, the number of devices for lighting a plurality of discharge-lamps connected in parallel has increased.
Japanese Patent Application Publication 2004-241136 discloses a discharge-lamp control device for a single discharge-lamp having two electrodes. The discharge-lamp control device includes a pair of inverters, each of which is electrically connected to each of two electrodes. In this apparatus, the lighting of the discharge-lamp is controlled by transmitting high-frequency alternating-current power from the inverter to the discharge-lamp.
However, when this apparatus is used to light a plurality of discharge-lamps connected in parallel, the pair of inverters are necessary for each discharge-lamp, which increases power consumption and manufacturing cost. In order to solve these problems, a new system for lighting the plurality of discharge-lamps has been developed and is commercially available which has two inverter circuits, and two driving circuits connected to each of the inverter circuits. In this system, one inverter is connected to one of the two electrodes of the plurality of discharge-lamps connected in parallel.
However, when the plurality of discharge-lamps connected in parallel is lighted in the above new system, the amount of electric power supplied from the driving circuits to the discharge-lamps may become unbalanced because of variation in the impedances of the discharge-lamps. The power balance may also lose by distributed capacities of the discharge-lamps induced by the alternating-current driving. When the power of the driving circuits is unbalanced, a variation in a current flowing in the discharge-lamp may arise, which may result in shortening the service lives of discharge-lamps.
As described above, variation in the impedances of driving circuits may result in a loss of the power balance and/or current balance of the driving circuits. Therefore, the above phenomenon may lead to variation in the brightness of the discharge-lamp along the longitudinal direction and/or shortening the service lives of the discharge-lamps.
One attempt to conform the impedances of the driving circuits is to mount another component for adjustment, such as a transformer and a ballast capacitor, in the driving circuit. However, it is still difficult to obtain power balance and current balance of the driving circuits because of initial variations in characteristics of these components.
Further, if the transformer and the capacitor are selected with more strict specifications, cost will increase for selecting the components, thereby increasing the manufacturing cost of the discharge-lamp control device.